In optical inspection technology, complex structures are inspected. This is the case for example in the semiconductor industry for the optical inspection of PCBs and wafers, and in the optics industry for the inspection of optical elements. Many inspection technologies exist, with 2D as well as with 3D capabilities. The chosen technology depends among others on the type of object and the type of searched information.
Some representative patent publications in the field include:
U.S. Pat. No. 6,556,706, to Geng, describes a three-dimensional imaging method and system illuminating an object to be imaged with a light pattern that is formed from two or more light sub-patterns. The sub-patterns can each encompass the visible light spectrum or can be spatially varying intensity sub-patterns that each correspond to a red, green, or blue component. The light pattern is generated by a slotted planar member or an optical filter. It should be noted that the system of '706 is not suitable for inspection and measurement of specular surfaces.
U.S. Pat. No. 5,825,476 to Abitbol et al., based on Hartman-Shack wavefront analysis method for transparent and specular objects, describes an apparatus for mapping an optical element. The teachings of Abitbol et al., comprises systems with low spatial resolution. Such systems are designed to measure dispersed sample points on the object under test and cannot be used to inspect objects continuously along their surface.
US 20060044988 to Laborelli, describes equipment for the optical playback of sound media which includes resources that have engraved grooves, for generating at least one light beam that presents a light spectrum variation according to its incidence angle on an area of the sound media and an image sensor placed so that it recovers the light reflected by said area of the sound media. This invention, applicable to specular surfaces, is limited in the shape of the measured area (line) and in its dimensions (same order of magnitude as the light source). Moreover, each point of the tested object sees a slightly different angular distribution.
US2004184031 discloses a three-dimensional optical inspection system, which reconstructs a three-dimensional image of the shape of the surface of an at least partially specular object resident on a printed circuit board by capturing two or more two-dimensional images of the object under different illumination configurations. The diffuse reflection, as well as the specular reflection can be used to reconstruct the three-dimensional image using any reconstruction method, such as photometric stereo. The different illumination configurations can be achieved using an illumination source including light-emitting elements arranged in concentric circular arrays, in which each of the circular arrays is divided into sections. Each section is independently controlled to selectively activate the sections to illuminate the object in a pre-established illumination pattern. The systems taught in '031 are limited in that they require a very large number of illumination sources and that there are dark regions dispersed between the illumination sources leading to “only a few discrete values of surface tilt”. Moreover, each point of the tested object sees a slightly different angular distribution, which adds uncertainty to the height measurement.
There is still a need to provide improved optical systems for surface metrology, having the advantages of the cited publications altogether, without their limitations.